1. Field of the Invention
The present invention generally relates to processes for shaping catalytic material as small spherical particles. The invention also relates to particulate catalysts and catalyst supports prepared by such process.
2. Description of Related Art
The physical and structural properties of a catalyst significantly influence its activity and durability. In many cases, the pore structure of the catalyst support, including size distribution and volume, determines the extent and accessibility of surface area available for contact of the catalytic material and the reactants. Catalytic activity often depends on the rate of diffusion of reactants and products in and out of the interstices of a catalyst. Increased pore size may facilitate the diffusion of reactants and reaction products, but catalytic activity is also a function of surface area and packing density, particularly with fixed bed particular catalysts. Spherically shaped catalyst particles have certain advantages over other shapes because they permit uniform packing so that variations in pressure drop are minimized and the tendency of a reactant stream to channel through the bed without effectively contacting the catalyst is reduced.
After a significant amount of use particulate catalysts and catalyst supports employed in a number of chemical processes tend to break down to smaller particles or fines. This is particularly problematic in fluid catalyst bed systems, where impacts experienced by the particles result in surface abrasion that produces fine particles which can be entrained in the stream of reactants. This usually contributes to a reduction in catalyst activity due to a loss of catalyst in the reactor. Better flow properties are generally obtained with spherical catalyst particles and catalyst attrition tends to be lessened compared to irregularly or non-spherically shaped particle beds.
U.S. Pat. No. 4,318,896 (Schoenover) discusses five general methods of preparing spheroidal particles of a size suitable for commercial operation. Of these, the spray drying method and the method of dropping particles into an oil bath are widely used. According to those methods, drops of a catalyst-forming liquid are produced and allowed to harden. In conventional spray drying techniques the droplet hardening takes place in a stream of air or in a water immiscible liquid such as oil.
A process for manufacturing silica particles is shown in U.S. Pat. No. 3,872,217. Processes for manufacturing alumina particles are shown, for example, in U.S. Pat. No. 4,318,896, and processes for manufacturing silica-alumina particles are described in U.S. Pat. No. 3,986,978. U.S. Pat. No. 2,620,314 (Hoekstra) describes a method for preparing a catalyst support, especially spheroidal alumina particles, by the oil-drop method.
U.S. Pat. No. 4,628,040 (Green) describes a method of making uniform spheroidal catalyst beads in which uniform droplets of a bead-forming liquid are produced by positioning the end of a capillary tube in the throat of a venturi. An immiscible fluid flowing through the venturi detaches the droplets from the end of the capillary tube to produce uniform, spherical droplets which harden into spheroidal beads of uniform size. This is contrasted with other oil-drop methods that initially form irregularly-shaped, non-uniformly sized particles which subsequently assume a spherical shape in the hot oil bath due to surface tension forces. Beads of alumina, silica alumina, and silica of about 200 microns or larger, up to xe2x85x9 inch in diameter are disclosed and compared to typical beads produced in spray drying, which are said to have a diameter of 20 to 150 microns.
U.S. Pat. No. 4,902,666 (Rainis) describes a process for the manufacture of spheroidal bodies by selective agglomeration. These spheroidal particles can have either smooth surfaces or polylobe surfaces depending on the conditions of preparation. They have diameters generally between 1 to 5 mm and are useful as catalysts, or catalyst supports.
U.S. Pat. No. 5,710,093 (Rivas) describes a catalyst support comprising spherical particles of a mixture of at least two refractory inorganic oxides, refractory inorganic carbides, refractory inorganic nitrides, and mixtures of those compounds. The particles have a surface area of at least about 30 m2/g, an average pore diameter of at least about 150 xc3x85, and a particle size of at least about 0.1 mm.
U.S. Pat. No. 4,766,101 (Nortier, et al.) describes certain alumina-based catalyst carriers in the form of particles such as spheres, pellets, extrudates and crushed material. The durability of the carriers is improved by stabilizing them by impregnation with an aqueous solution containing silicon, in the form of the silicate ion, and nitrogen in the form of a quaternary ammonium ion, and then drying and activating the impregnated carriers by a calcination which decomposes the organic cation into volatile compounds which diffuse out of the carrier.
U.S. Pat. No. 5,877,381 (Sasaki, et al.) discusses the importance of maintaining a certain particle size distribution of the catalyst in order to maintain a good fluidized state in fluidized bed reactions for syntheses of organic compounds. It is suggested that the catalyst particles tend to be crushed or worn more easily if the particles have a finer particle size, so that the strength of particles having a smaller diameter is particularly important in reducing catalyst loss.
One type of industrial process in which particulate catalysts are employed is in a conventional Fischer-Tropsch process, in which carbon monoxide and hydrogen are converted via an exothermic reaction to the desired C2+ hydrocarbon end products. The CO and H2 reactant gas mixture is referred to as xe2x80x9csyngas.xe2x80x9d The types and amounts of reaction products, i.e., the lengths of carbon chains, obtained via Fischer-Tropsch synthesis vary dependent upon process kinetics and the choice of catalyst. Slurry phase reactions, particularly those occurring in bubble columns are well-known in the art and have been thoroughly described in the literature for carrying out Fischer-Tropsch hydrogenation reactions. See, for example, Farley et al, The Institute of Petroleum, Vol. 50, No. 482, pp. 27-46, February (1984). In a three-phase slurry reactor a fluidized gas is introduced into a reactor containing catalyst particles slurried in liquid hydrocarbons within a reactor chamber, which is typically a tall column. Syngas is then introduced at the bottom of the column through a distributor plate, which produces small gas bubbles. The gas bubbles migrate up and through the column, causing a beneficial turbulence, while reacting in the presence of the catalyst to produce liquid and gaseous hydrocarbon products. Gaseous products are captured at the top of the reactor, while liquid products are recovered through a filter that separates the liquid hydrocarbons from the catalyst fines.
A variety of catalysts have been described in the literature for enhancing the efficiency and selectively of syngas to liquid hydrocarbons. One common type of catalyst used in Fischer-Tropsch synthesis is a cobalt-based catalyst prepared by loading of the catalytic material on a support using impregnation by incipient wetness or other well known techniques. For example, a titania, silica or alumina support may be impregnated with a cobalt nitrate salt solution, optionally followed or preceded by impregnation with a promoter material. Excess liquid is removed and the catalyst precursor is dried. Following drying, or during continued drying, the catalyst is calcined to convert the salt and promoter to the corresponding metal oxide(s). The oxide is then reduced by treatment with hydrogen or a hydrogen-containing gas for a period of time sufficient to substantially reduce the oxide to the elemental or catalytic form of the metal. Most conventional catalyst production methods do not provide uniform, spherical particles in the micrometer diameter size range (i.e., from less than 1 micrometer up to about 1000 micrometers), especially in the quantities needed for industrial scale use.
U.S. Pat. No. 6,100,304 (Singleton, et al.) describes cobalt and a palladium promoter supported on gamma-alumina or doped gamma-alumina for catalyzing Fischer-Tropsch synthesis in a slurry bubble column reactor. The source of the alumina and the pretreatment procedures used are said to play major roles in determining the performance of the resulting cobalt-based Fischer-Tropsch catalysts. The disclosed spheroidal shaped alumina supports have an average particle size ranging from about 10 to about 150 xcexcm, a BET surface area, after calcination, ranging from about 200 to about 260 m2/g; and may include about 0-1000 ppm titanium added prior to crystallization. The resulting cobalt-based catalysts are said to be much more attrition resistant than cobalt catalysts utilizing other types of oxide supports (e.g, silica), even when those other supports are spheroidal. Additional improvement in attrition resistance is said to be obtained by incorporating a lanthana (La2O3) promoter.
U.S. Pat. No. 5,252,613 (Chang et al.), which describes a method for obtaining enhanced catalyst mixing in slurry bubble columns, states that catalyst particle sizes may range from that which is reasonably filterable to that which is reasonably able to be dispersed in a slurry phase. Particle sizes of 1-200 microns, preferably about 20 to 150 microns are said to meet these requirements.
Despite the prior art disclosures there still remains a need for a practical method of producing small, spherical particles for use in applications requiring micrometer diameter range particles with enhanced resistance to abrasion. One such application is in the manufacturing of more catalytically active, attrition resistant particles for use in catalyzing chemical reactions such as the hydrogenation of carbon monoxide in a slurry bubble column Fischer-Tropsch process.
The present invention overcomes many of the problems encountered with prior art particles that are employed as catalysts or catalyst supports, and provides a new method of making spherical or substantially spherical particles that are especially suited for use as catalysts and catalyst supports. As used herein, the term xe2x80x9csubstantially sphericalxe2x80x9d means regularly shaped, rounded particles that resemble spheres. According to certain embodiments of the invention, a new method for producing small spherical particles that are especially useful as attrition resistant catalysts and catalyst supports employed in chemical processes is provided. In accordance with certain embodiments of the invention, a method of making at least one spherical particle comprises loading or impregnating a support with a metal and/or metal oxide and then dissolving the support, whereby separate spherical particles are released. The support comprises a material that is labile to a selected treatment, and the catalytic metal or metal oxide is stable, or at least substantially stable, to the same treatment, which in certain embodiments, is an acid or alkali treatment. As used herein the term xe2x80x9csubstantially stablexe2x80x9d means that the catalyst metal or metal oxide does not chemically or physically deteriorate under the specified treatment conditions over a specified period of time. The support may be in the form of particulates or a monolith prior to the support-dissolving treatment, and contains at least one spherical or substantially spherical void. In some embodiments, there are voids in the support with a diameter in the range of about 0.1 to 10 microns, preferably about 2 microns. In certain embodiments the method also includes treating the loaded support with an alkali or an acid.
Also in accordance with the invention is provided a spherical particle having a micrometer range diameter, comprising at least one metal or metal oxide, and prepared as described above. In certain preferred embodiments the spherical particle is an attrition resistant catalyst particle or particulate catalyst support made by impregnating a support with a solution comprising at least one decomposable metal salt, to yield a supported catalyst precursor. The support contains a plurality of spherical or at least substantially spherical voids. The catalyst precursor is then calcined and, optionally, may be reimpregnated with a second solution comprising at least one decomposable metal salt and calcined. The support is then dissolved in such a way that a plurality of spherical metal and/or metal oxide particles having activity for catalyzing a defined chemical reaction are released from the support. The free spherical particles are then harvested for use as a catalyst or catalyst support. In certain preferred embodiments the method of making the catalyst or catalyst support particle comprises choosing a support having a number of spherical or substantially spherical voids, such as a silica-containing material.
Certain embodiments of the invention provide a particulate catalyst comprising a plurality of micrometer range diameter spherical particles prepared by impregnating a support with a metal or metal oxide having activity for catalyzing a desired chemical reaction. Preferably the support that is chosen comprises a plurality of substantially spherical voids. The impregnated support is then dissolved, releasing the particles which, in certain embodiments comprise at least about 10% spherically shaped particles. In some embodiments the diameter ranges from about 0.1 to 10 microns, and in some preferred embodiments the particles have an average diameter of about 2 microns.
Also provided by the present invention are chemical processes employing the new spherical catalyst particles or catalyst supports. One such process comprises a process for synthesizing C2+ hydrocarbon products comprising contacting a feedstock comprising CO and H2 with a particulate catalyst under Fischer-Tropsch hydrogenation reaction promoting conditions. The catalyst comprises a plurality of spherical particles prepared by impregnating a support with a solution comprising at least one decomposable metal salt, to yield a supported catalyst precursor. The catalyst precursor is then calcined and, optionally, may be reimpregnated with a second solution comprising at least one decomposable metal salt and calcined. The support is then dissolved in such a way that a plurality of spherical metal and/or metal oxide particles having activity for catalyzing a defined chemical reaction are released from the support. The free spherical particles are then harvested for use as a particulate catalyst. In certain alternative embodiments, the spherical particles themselves have little or no catalytic activity in the Fischer-Tropsch reaction, in which case the particles serve as a catalyst support for a more catalytically active material that is applied to the particles. Still other embodiments and advantages of the present invention will appear from the following description.